Mechanisms of the polyol reduction of copper(ii) salts depending on the anion type and diol chain length

Abstract

The mechanisms of the polyol reduction of copper(II) compounds were investigated by various systematic experiments. The course of the reduction in ethylene glycol strongly depends on the anion present in solution. Elemental copper can only be obtained in high yields starting from Cu(NO₃)₂, Cu(OAc)₂, or Cu(OH)₂; but not from CuCl₂ or CuSO₄. Intermediate compounds were observed, namely Cu₂O, and the alkoxide compounds Cu(C₂H₄O₂) and Cu₃(OAc)₂(C₂H₄O₂)₂. Cu₃(OAc)₂(C₂H₄O₂)₂ was characterised by single-crystal X-ray diffraction. It is a 2D coordination polymer with bridging bidentate acetate and bidentate deprotonated EG ligands leading to vertex- and edge-linked CuO₅ polyhedra. Alkoxide compounds were also detected for higher glycol ethers. With growing glycol ether chain length the stability of alkoxide intermediates decreases, enhancing the stability range of Cu₂O. Because the majority of copper species is precipitated as copper(II) alkoxide compounds or Cu₂O before the reduction to elemental copper occurs, only few nuclei are formed in solution leading to microparticles instead of nanoparticles. Much smaller but strongly agglomerated particles were observed under alkaline reaction conditions.